With the enhancement of speed of and the reduction in the price of wireless LAN systems, recently, the demand for them has significantly grown. Especially these days, the introduction of personal area network (PAN) has been widely considered to build a small-scale wireless network among a plurality of pieces of electronic equipment common around the house for information communication. For example, different radio communication systems have been defined using frequency bands, such as 2.4-GHz band and 5-GHz band, for which licenses from competent authorities are unnecessary.
In radio communication including wireless LAN, information is transmitted through antennas. For example, a monoconical antenna comprises a radiation electrode formed in a substantially conical concavity in a dielectric, and a ground electrode formed on the bottom face of the dielectric. Thus, a small antenna having relatively wideband characteristics can be constituted by the wavelength shortening effect from the dielectric positioned between the radiation electrode and the ground electrode.
An antenna having wideband characteristics can be used in UWB (Ultra-Wide Band) communication wherein, for example, data is spread in as ultra-wide a frequency band as 3 GHz to 10 GHz for transmission and reception. A small antenna contributes to reduction in the size and weight of radio equipment.
For example, Japanese Unexamined Patent Publication No. Hei 8(1996)-139515 discloses a small dielectric vertical polarization antenna for wireless LAN. This dielectric vertical polarization antenna is constituted as follows: one base of a cylindrical dielectric is conically hollowed out, and a radiation electrode is formed there, and an earth electrode is formed on the base on the opposite side. The radiation electrode is drawn out to the earth electrode side through a conductor in a through hole. (Refer to FIG. 1 in the Unexamined Patent Publication.)
FIG. 5 in the Unexamined Patent Publication illustrates the antenna characteristics of this dielectric vertical polarization antenna. According to the figure, its operating band is approximately 100 MHz. (The center frequency is approximately 2.5 GHz; therefore, the relative bandwidth is approximately 4%.) The monoconical antenna has inherently an operating band not less than one octave; therefore, it cannot be said that the above antenna sufficiently delivers expected wideband characteristics.
The miniaturization of an antenna means reduction in, for example, its profile or width. For example, Japanese Unexamined Patent Publication No. Hei 9(1997)-153727 presents a proposal with respect to reduction in the width of monoconical antenna. However, the proposal is such that a radiation conductor should be simply formed in the shape of semi-elliptic solid of revolution, and whether it is applicable to the structure of an antenna whose side face is covered with dielectric without any modification is unknown.
FIG. 31 schematically illustrates the constitution of a monoconical antenna having a single conical radiation electrode. The monoconical antenna illustrated in the figure comprises a radiation conductor formed in substantially conical shape, and a ground conductor formed with a gap provided between it and the radiation conductor. Electrical signals are fed to the gap.
FIG. 32 illustrates an example of the VSWR (Voltage Standing Wave Ratio) characteristics of a monoconical antenna. A VSWR not more than 2 is attained over a wide range from 4 GHz to 9 GHz, and this indicates that the antenna has a wide relative bandwidth.
One of known methods for further widening the band of this monoconical antenna is loading resistance on the radiation conductor. FIG. 33 and FIG. 34 illustrate examples of the constitutions of monoconical antennas whose radiation conductor is formed of a low-conductivity member containing a resistance component, instead of high-conductivity metal. With this constitution, reflective power to a feeding portion is diminished, and this results in expanded matching band. Especially, since the lower limit frequency of the matching band is expanded (downward), the above constitutions are also utilized as means for the reduction of antenna size. As illustrated in FIG. 33, the radiation electrode may be formed of a material having a constant low conductivity. However, if the conductivity is distributed as illustrated in FIG. 34 (lower conductivity on the upper base side), the effect is produced better.
Various methods are known for loading resistance on the radiation conductor of a monoconical antenna. Concrete examples include a method of sticking a low-conductivity member formed in sheet shape to a conical insulator, and a method of applying a low-conductivity member prepared as coating material. (Refer to “Optimization of a Conical Antenna for Pulse Radiation: An Efficient Design Using Resistive Loading,” written by James G. Maloney, et al. (IEEE Transactions on Antennas and Propagation, Vol. 41, No. 7, July, 1993, pp. 940-947), for example.)
However, if mass production is considered, the method of sticking a sheet is indeed inferior in productivity, and is not realistic. With the method of applying coating, it is difficult to make the thickness of coating uniform to control conductivity, and this method is also unrealistic.